1. Field of the Invention
The present invention relates to polymerization catalysts containing titanium and vanadium. The activated catalyst system produces olefin polymers having broad molecular weight distribution and multimodal characteristics. Importantly, the catalysts of the present invention permit the formation of these olefin polymers in a single stage reactor, thus avoiding problems due to undispersed polymer resin and eliminating the need for a cascade reactor system. The present invention also relates to the polymerization process using these catalysts and the polymers thus produced.
2. Description of the Prior Art
The polymerization of olefins using transition-metal based catalysts is well established in the art. Catalysts of this type which are useful in low temperature, low pressure polymerization processes--particularly those involving ethylene homopolymerization or co-polymerization, have been the subject of much research. Of special interest is the development of a catalyst which will yield a polymer with broad molecular weight distribution as manifested, for example, by a bimodal characteristic. Such a polymer would be particularly adaptable for use in high strength films and light weight blow molding resins because as molecular weight distribution broadens, i.e., as the lowest and highest molecular weights become further apart, film strength and resin processability increase.
To achieve these goals, catalysts comprised of three transition metals have been investigated. One such attempt is U.S. Pat. No. 4,710,552 which discloses a Ziegler catalyst system comprising an organoaluminum component, an organohalogen component and a transition metal catalyst component. The transition metal catalyst component involves an inorganic oxide support, a saturated alicyclic oxahydrocarbon, a vanadium trichloride/alcohol complex, a titanium trihalide or titanium trihalide/alcohol complex and, optionally, a zirconium tetrahalide.
Similarly, U.S. Pat. No. 4,831,090 provides a Ziegler catalyst wherein the transition metal component is prepared from an inorganic oxide substance (used as a carrier), an oxahydrocarbon, a vanadium trichloride/alcohol complex, a titanium trihalide or titanium trihalide/alcohol complex and, optionally, a zirconium tetrahalide. Other components include an organoaluminum and an organohalide.
The above remarks establish that there is a continuing need in the art for catalysts useful in the production of olefin polymers having a broad molecular weight distribution and multimodal characteristics in a single reactor, where the polymers thus produced have improved stereoregularity, uniform particle size distribution, good spherical morphology, and the catalysts themselves manifest excellent catalytic activity and response to hydrogen.